1. Field of the Invention
The invention relates to aqueous coating compositions, their preparation, and their use.
2. Description of Related Art
Since it is necessary, in order to reduce and avoid pollution, to bring about a continual reduction in the content of organic solvents, aqueous coating compositions are increasingly being employed for all functions in enhancing the quality of surfaces.
When solvent-containing coating systems are replaced by solvent-free or low-solvent systems, the advantageous performance properties of the previous solvent-containing systems and their good protective action must be retained or, where possible, exceeded.
To prepare the individual binder components of the coating compositions, it is frequently necessary for reasons of viscosity or reactivity, to employ solvents. Thus, in the synthesis of the base resins, it is preferred to use protic, such as alcohols and glycols, and aprotic solvents, such as ketones, esters, or aromatic compounds, and to use aprotic solvents in the synthesis of the crosslinking component.
As is known, however, an increase in solvent content leads, for example, in cathodic deposition coating baths, to a deterioration in the throwing power (coating of cavities), to instances of excess coating and to the increased passage of solvent into the waste water and/or the waste air. As a consequence it is necessary to remove the solvents from the binders before and/or after the step of dispersion in water, for example, by distillation or ultrafiltration. With these methods, some of the solvent remains in the dispersions, and, especially in the case of so-called stripping from the aqueous phase and in the case of ultrafiltration, relatively large quantities are produced of a solvent/water mixture which has to be disposed of or reprocessed.
The crosslinking agents of the prior art are present as a solution in organic solvents or as an aqueous dispersion, but a considerable proportion (in a typical example about 30% of the mass of the dispersion) of solvents must remain in the dispersion. This quantity of solvent is the largest component in the finished coating system which comprises, for example, binder, curing, agent and pigment paste.